fix: hash inputs for external nullifier, remove length prefix for sha256 (#3660)

* fix: hash inputs for external nullifier, remove length prefix for sha256

* feat: use nimcrypto keccak instead of sha256 ffi

* feat: wrapper function to generate external nullifier

tests/waku_rln_relay/test_waku_rln_relay.nim

@@ -70,53 +70,6 @@ suite "Waku rln relay":
 
     info "the generated identity credential: ", idCredential
 
-  test "hash Nim Wrappers":
-    # create an RLN instance
-    let rlnInstance = createRLNInstanceWrapper()
-    require:
-      rlnInstance.isOk()
-
-    # prepare the input
-    let
-      msg = "Hello".toBytes()
-      hashInput = encodeLengthPrefix(msg)
-      hashInputBuffer = toBuffer(hashInput)
-
-    # prepare other inputs to the hash function
-    let outputBuffer = default(Buffer)
-
-    let hashSuccess = sha256(unsafeAddr hashInputBuffer, unsafeAddr outputBuffer, true)
-    require:
-      hashSuccess
-    let outputArr = cast[ptr array[32, byte]](outputBuffer.`ptr`)[]
-
-    check:
-      "1e32b3ab545c07c8b4a7ab1ca4f46bc31e4fdc29ac3b240ef1d54b4017a26e4c" ==
-        outputArr.inHex()
-
-    let
-      hashOutput = cast[ptr array[32, byte]](outputBuffer.`ptr`)[]
-      hashOutputHex = hashOutput.toHex()
-
-    info "hash output", hashOutputHex
-
-  test "sha256 hash utils":
-    # create an RLN instance
-    let rlnInstance = createRLNInstanceWrapper()
-    require:
-      rlnInstance.isOk()
-    let rln = rlnInstance.get()
-
-    # prepare the input
-    let msg = "Hello".toBytes()
-
-    let hashRes = sha256(msg)
-
-    check:
-      hashRes.isOk()
-      "1e32b3ab545c07c8b4a7ab1ca4f46bc31e4fdc29ac3b240ef1d54b4017a26e4c" ==
-        hashRes.get().inHex()
-
   test "poseidon hash utils":
     # create an RLN instance
     let rlnInstance = createRLNInstanceWrapper()

waku/waku_rln_relay/group_manager/on_chain/group_manager.nim

@@ -379,7 +379,7 @@ method generateProof*(
 
   let x = keccak.keccak256.digest(data)
 
-  let extNullifier = poseidon(@[@(epoch), @(rlnIdentifier)]).valueOr:
+  let extNullifier = generateExternalNullifier(epoch, rlnIdentifier).valueOr:
     return err("Failed to compute external nullifier: " & error)
 
   let witness = RLNWitnessInput(
@@ -457,10 +457,9 @@ method verifyProof*(
 
   var normalizedProof = proof
 
-  normalizedProof.externalNullifier = poseidon(
-    @[@(proof.epoch), @(proof.rlnIdentifier)]
-  ).valueOr:
+  let externalNullifier = generateExternalNullifier(proof.epoch, proof.rlnIdentifier).valueOr:
     return err("Failed to compute external nullifier: " & error)
+  normalizedProof.externalNullifier = externalNullifier
 
   let proofBytes = serialize(normalizedProof, input)
   let proofBuffer = proofBytes.toBuffer()

waku/waku_rln_relay/rln/wrappers.nim

@@ -6,7 +6,8 @@ import
   stew/[arrayops, byteutils, endians2],
   stint,
   results,
-  std/[sequtils, strutils, tables]
+  std/[sequtils, strutils, tables],
+  nimcrypto/keccak as keccak
 
 import ./rln_interface, ../conversion_utils, ../protocol_types, ../protocol_metrics
 import ../../waku_core, ../../waku_keystore
@@ -119,24 +120,6 @@ proc createRLNInstance*(): RLNResult =
     res = createRLNInstanceLocal()
   return res
 
-proc sha256*(data: openArray[byte]): RlnRelayResult[MerkleNode] =
-  ## a thin layer on top of the Nim wrapper of the sha256 hasher
-  var lenPrefData = encodeLengthPrefix(data)
-  var
-    hashInputBuffer = lenPrefData.toBuffer()
-    outputBuffer: Buffer # will holds the hash output
-
-  trace "sha256 hash input buffer length", bufflen = hashInputBuffer.len
-  let hashSuccess = sha256(addr hashInputBuffer, addr outputBuffer, true)
-
-  # check whether the hash call is done successfully
-  if not hashSuccess:
-    return err("error in sha256 hash")
-
-  let output = cast[ptr MerkleNode](outputBuffer.`ptr`)[]
-
-  return ok(output)
-
 proc poseidon*(data: seq[seq[byte]]): RlnRelayResult[array[32, byte]] =
   ## a thin layer on top of the Nim wrapper of the poseidon hasher
   var inputBytes = serialize(data)
@@ -180,9 +163,18 @@ proc toLeaves*(rateCommitments: seq[RateCommitment]): RlnRelayResult[seq[seq[byt
     leaves.add(leaf)
   return ok(leaves)
 
+proc generateExternalNullifier*(
+    epoch: Epoch, rlnIdentifier: RlnIdentifier
+): RlnRelayResult[ExternalNullifier] =
+  let epochHash = keccak.keccak256.digest(@(epoch))
+  let rlnIdentifierHash = keccak.keccak256.digest(@(rlnIdentifier))
+  let externalNullifier = poseidon(@[@(epochHash), @(rlnIdentifierHash)]).valueOr:
+    return err("Failed to compute external nullifier: " & error)
+  return ok(externalNullifier)
+
 proc extractMetadata*(proof: RateLimitProof): RlnRelayResult[ProofMetadata] =
-  let externalNullifier = poseidon(@[@(proof.epoch), @(proof.rlnIdentifier)]).valueOr:
-    return err("could not construct the external nullifier")
+  let externalNullifier = generateExternalNullifier(proof.epoch, proof.rlnIdentifier).valueOr:
+    return err("Failed to compute external nullifier: " & error)
   return ok(
     ProofMetadata(
       nullifier: proof.nullifier,